Atmospheric ImbalanceAtmospheric gases are non-toxic but alterations in their concentration, especially of oxygen, have a direct effect upon life and combustion processes.

It is essential to have sufficient oxygen in atmospheres being breathed.

Although not flammable itself, oxygen does support combustion, whereas nitrogen and argon inhibit combustion.

If good practice is not observed, accidents may happen since changes in concentration cannot be detected in good time by the human senses.

Volumetric composition of air

An understanding of the approximate volumetric composition of the main components of air can be helpful in preventing accidents.

They are as follows:

Gas

Symbol

%vol

Oxygen

O2

21

Nitrogen

N2

78

Argon

Ar

1

Atmospheric imbalance: welding and heating processes

The use of welding and heating processes can lead to an imbalance in atmospheric gases. This can result in the following hazardous conditions:

oxygen enrichment

oxygen deficiency

fuel gas accumulation

toxic fume build up

Oxygen Enrichment

Definition

'Oxygen enrichment' is the term used to describe an atmosphere which has a higher percentage of oxygen than 'normal' air.

The oxygen content of normal air is 21%, but a small increase in oxygen can have a dramatic effect.

Causes

leakage of oxygen gas

spillage of liquid gases

incorrect storage of vessels or open dewars

Dangers

When materials burn they consume oxygen. When oxygen levels in the prevailing atmosphere are normal materials will burn normally.

If oxygen levels become elevated above the normal 21%, flammability behaviour can change significantly. Materials which are not normally flammable may become so. Those materials which are flammable in normal atmospheric conditions can burn much more quickly and aggressively.

For example, overalls treated to be 'flame-retardant' can, in an oxygen enriched atmosphere, burn vigorously.

Oxygen Deficiency

Definition

'Oxygen deficiency' occurs when an atmosphere has an oxygen level which is lower than 'normal'.

Normal air contains 21% oxygen at sea level.

Causes

leakage of gases other than oxygen

spillage of liquid gases

corrosion processes consuming oxygen

incorrect storage of vessels or open dewars

welding and heating processes. All gas welding and heating processes take oxygen from the air and can cause a deficiency unless the size of the workspace and its ventilation are sufficient

Dangers

Oxygen is essential to life. It is vital to ensure that adequate oxygen is present in any atmosphere being breathed.

While a healthy person may survive a short exposure to an oxygen content as low as approximately 16%, no one should ever be asked to endanger his or her life by breathing such an atmosphere.

A perilous feature of oxygen deficiency is that it cannot readily be detected by the senses. Victims are usually unaware of the danger they are in and may even have a feeling of well-being.

Fuel gas accumulation

Danger

Fuel gas accumulation is extremely dangerous as, when mixed with an oxidant and provided with an ignition source, it will burn.

Causes

Ignition occurs within a range of concentrations.

The lower limit is particularly important as that is the one reached first.

There is hardly any danger that these ignition limits will be reached in large welding shops which have good ventilation, or even inside containers or confined spaces in the open air.

However, even small quantities of escaping fuel gas are sufficient, under certain conditions, to form an ignitable mixture.

Bear in mind that even if a fuel gas has quite a tight flammable mixture range in air, at higher concentrations (unless the vapour cloud is entirely homogeneous) there are still likely to be areas of the cloud with a flammable composition.

Fortunately acetylene can often be recognised by its distinctive garlic-like smell in very small concentrations of well below 2%. Acetylene in the air is therefore very easily detected by a good sense of smell, allowing corrective action to be taken.

Dangers

Escaping fuel gases may form mixtures with the surrounding air, leading to fires and explosions.

Because of its high specific gravity, propane flows to the ground like a liquid when there is little air movement. In such conditions it may remain there for a very long time.

It can accumulate in pits, ducts and trenches. It is then possible for ignitable concentrations to arise, sometimes at a distance from the leakage source.

Toxic fume build up

Definition

Toxic fume build up is the build up of gases arising during welding and cutting processes.

Welding fume is an unavoidable by-product of welding. It consists of particulate fume (the part you can see) and gaseous fume (which you cannot see but can sometimes smell).

All welding processes generate welding fume. The type and amount varies from process to process; MMA, MIG, FCAW, and plasma cutting tend to produce most, while TIG, plasma welding, laser welding and Submerged Arc give very little under normal circumstances, although TIG can give high levels of gaseous fume.

Causes

Cutting and welding metals by flame or arc processes produces fume. The fume is comprised of two parts:

particulate fume. Usually visible as smoke consisting of fine metal oxide powder. The composition of this fume depends upon the metal being cut or welded and, in the case of welding, the composition of the filler rod or wire

gaseous fume. Non-visible, usually composed of combinations of oxides of carbon, oxides of nitrogen and ozone

Dangers

It is important to understand that welding fume is hazardous to operators.

For further information please consult your local Health and Safety Executive Office.

The overall OES for welding fume sets a maximum concentration of 5mg of fume per cubic metre of air.

Preventing atmospheric imbalance

Ensure good ventilation

Wherever cylinder gases are used, constant and thorough ventilation should be maintained. This is particularly important when they are used in confined spaces.

In order to avoid exposure to fumes, general and local extraction is essential. Local ventilation should be arranged to suck the fume away as it is formed. A number of companies market such units.

General ventilation should ensure that the entire atmosphere of the workshop is changed a number of times during a shift. A range of fume extraction equipment is available from BOC trade outlets.

Take special precautions when welding or cutting coated material as their coatings can produce toxic fumes.

Avoid leaks

Assemble equipment carefully

Ensure that valves are closed when apparatus is not in use. Hazardous situations can arise through negligence such as failure to close valves. For example, leaky equipment or torches with the fuel gas valve not properly closed have been left in small shops for long intervals, and when work was resumed serious accidents occurred.

Conduct leak testing

It is very important to observe the correct use of blowpipes. Care should be taken, especially in confined spaces, to avoid delay in lighting the blowpipe after opening the valves.

When flame cutting, besides the oxygen required for the preheating flame, a considerable amount of oxygen is required for burning the material and blowing out the slag. This leads to an excess of unused oxygen, the amount of which will increase if the pressure employed is too high, or if the nozzle is too big for the workpiece being cut.

This indicates the importance of selecting the correct nozzles and pressures.

Remove gas equipment from confined spaces when not in use

Gas equipment connected to a supply must not remain in confined spaces or vessels during intervals or meal breaks. They must be removed at that time.

Check the properties of filler rods and wires

Some filler rods and wires used in welding or brazing may contain particular toxic materials.

The suppliers of these rods or wires should be contacted regarding potential hazards and any special fume removal requirements.

Safety in the workplace:

The information contained herein is provided as a general outline and is not intended to be a definitive statement on the subject matter.

Professional advice should be sought before any action is taken in relation to safety in the workplace.